This invention relates generally to the art of helium ionization detectors and more specifically to improvements in helium ionization detectors.
The helium ionization detector (HID) is one of the most sensitive detectors currently available for gas chromatography. The detector is nonselective, meaning that it is capable of responding to all chromatographable species ranging from the permanent gases to complex organic molecules. Despite its universal response mechanism and high ionization efficiency, the HID detector has not been widely used. The reasons for its limited use include the stringent requirements for high sensitivity operation, instability, and variations in response for selected species as a function of chromatographic conditions. The greatest sensitivity is obtained when ultrapure helium is used as the carrier gas and when contributions from the chromatographic system (e.g., column bleed) to the background current are minimal. Low parts-per-billion concentrations of the permanent gases can be determined under these conditions.
The universal response characteristic which is largely an advantage can also be troublesome since any atmospheric diffusion in the system will reduce the sensitivity of the detector. Long periods of time may be required to stabilize the detector on initial start-up, when changing separation columns, or following any exposure of the system to the atmosphere. The response to the substrate or solvent used in the column may also be excessively large requiring long periods between sample analyses to allow the detector to return to initial background conditions. When the detector is overloaded by high concentrations of an analyte, or if the background is high, anomalous peak shapes or polarity inversions may be obtained making it difficult to interpret the results.
Despite these problems, there has been renewed interest in the HID. The characteristic negative responses for the permanent gases have been examined and the conditions which invert the signals defined. It was also determined that the detector could be operated in the saturation region of the field intensity with sensitivities comparable to those which can be obtained in the exponential region (i.e., at greater than 350 V). This is due to a decrease in noise level and background current. These reductions in turn have allowed gas-liquid partition columns to be used with the detector which extends the applications to include higher molecular weight organics.
Thus, it will be seen that there is a need for an improved HID system which provides stable operation of an HID while improving the sensitivity and linearity of the device.